Double-screw extruders and their housing are well known in the extruder art and are used to prepare and advance materials to be extruded through a die by rotation of the extruder screws in respective screw-receiving bores which can extend substantially over the length of the housing or a housing segment.
The term “housing segment” is here used to refer to an axially-extending portion of an extruder housing which can be joined to other such portions in axial alignment to form the entire housing. In other words the housing may be a one-piece housing whose bores extend over substantially the entire length of the extruder or a housing assembled from a plurality of such housing segments which can be joined together with screws. For this purpose, each segment can have a flange at an end thereof which is secured to the flange of another such segment or some other housing part by means of screws connecting the adjoining flanges together.
It is common to harden the wall of the screw-receiving bore in such housing or housing segment at least over high-wear segments of such walls. The hardening may be achieved by case hardening, nitriding, heat treatment or the like to an appropriate depth, i.e. radially outwardly of the bore.
When reference is made to the “housing” here, it is intended to thereby refer to the entire housing, e.g. a one-piece housing, or to a housing segment capable of being joined to other such segments to form the housing, unless the context indicates otherwise.
The housing of a double-screw extruder can be composed for example of heat-treatable nitride steel. Following the machining of the housing to form the screw-receiving bores to their final dimensions, the housing is usually subjected to heat treatment to the desired hardening level and thus to provide the requisite wear-resistant property.
With such double-screw extruder housings, this method has been found to be disadvantageous since, even with wear to a depth of only about 0.1 mm, the nitride hardening layer is consumed and the wear of the softer material therebelow can proceed significantly more rapidly. Such a housing or housing segment can no longer be regenerated and must be replaced.
One technique for overcoming this problem is to make the extruder housing with larger screw-receiving bores than is necessary to accommodate the screw. Then lining sleeves with a thickness of say 10 mm can then be inserted into the bores and can be composed of a wear-resistant material. When these sleeves are worn, they can be replaced. However, problems are then encountered with heat transfer since there is a resistance to thermal conduction between the lining sleeve and the housing since the sleeves are only pressed into the housing.
Different hardening methods have been described in the literature. For example, DE 10 048 870 C2 describes the introduction into the screw-receiving bores of a solid or hollow core and the filling of the space between the core and the housing with a metal/carbide powder mixture and the bonding of the particles of this mixture together and to the housing by a HIP process (Hot-Isostatic Pressing). In the HIP process, the HIP material in pulverulent form is compacted at a pressure in excess of 1000 bar and at a temperature upwards of about 1250° C. The hot isostatic pressing technique requires a metal housing which is welded to a hermetic seal which can be sustained even at such high temperatures. The metal capsule is filled and highly evacuated. The materials used can be metal alloys employed in powder metallurgy and whose alloy content will not support the loss of solubility and the phase diagrams. The material which is produced has the hardened or hardening layer and is free from segregations and has a fine-grained isotropic microcrystalline structure.
Metal/carbide powder mixtures and similar materials are relatively expensive and with this earlier process, it can be a drawback that the entire surface of each screw-receiving bore must be covered with the relatively expensive material.
A housing of the type described at the outset for a double-screw extruder has been described also in DE 29 913 216 U1 (see especially FIG. 5 thereof). This reference describes the concept of providing the wear-resistant materials only over a certain angular sector of each bore, i.e. in the region at which greatest wear can be expected. In a standard double-screw extruder, these regions can be the regions from 10 to 12 o'clock for the left screw-receiving bore as seen in cross section and the region from the 12 o'clock to 2 o'clock positions for the right-hand screw.
The wear-resistant material is in that case in the form of an insert which is set into the wall of the bore.
U.S. Pat. No. 5,752,770 describes the joining of a wear-resistant layer over part of the periphery of a bore with the remaining bore at a shoulder or junction.
With these earlier systems, there can be difficulties in fabricating the housing. For example, in DE 299 13 316 U1, the insert pieces are fabricated separately and are fastened in grooves of the housing by clamping screws. The fabrication of grooves over the length of the extruder-housing is also a problem as is the fastening of the inserts where they will be exposed to high forces. In U.S. Pat. No. 5,752,770, welding has been proposed and even that cannot be used with assurance since relatively thin and long bores for the extruder screws frequently will not permit welding to be used.